The volcano spewed a dirty-white grayish ash plume from a vent on its eastern slope at 7:36 a.m., said the Philippine Institute of Volcanology and Seismology (Phivolcs). The ash plume was one kilometer high, it added.

This alert level means more steam-driven eruptions are possible due to hydrothermal volcanic activity. The volcano erupted in June this year.

The public is prohibited from entering the "permanent Dangerous Zone," a four-kilometer radius around Bulusan, due to the possibility of eruptions.

Phivolcs said the steam explosion originated at Mapaso, Irosin, and affected the towns of Casiguran and Gubat due to the direction of the wind.

The Civil Aviation Authority of the Philippines has issued a notice to aircraft effective 9:00 a.m. Monday until 9:00 a.m. Tuesday, advising them to exercise extreme caution when flying in airspace near the volcano due to the volcanic ash cloud.

Residents living in valleys and nearby streams of the volcano's northwest and southwest flanks were advised to guard against sediment-laden flow and lahar due to rain.

Laguerta said a team is now assessing areas that may be affected.

Bulusan, which has four major craters, has erupted since 1978.

These craters have coalesced creating new craters around the volcano, said Laguerta.